In electrostatic printing it is generally required to render visible or develop a latent image defined by electrostatic charges contained on a dielectric or insulative surface of a recording member. Such recording member may be a photoconductor as used in the commonly known process of electrophotography or xerography, or a dielectric material as used in facsimile recording or computer printout and the like. The term electrostatic printing also applies to other methods of latent electrostatic image formation and rendering visible same, such as for instance to those methods in which a latent electrostatic image is formed on a surface by the so-called Dember effect, or by thermal means, or by physical means such as pressure or impact and the like.
The electrostatic latent images thus formed are rendered visible or developed in accordance with prior art practices by the application to the surface of electroscopic marking particles which are more or less selectively attracted to or repelled by the electrostatic charges defining the latent image, depending on whether a direct or a reversal reproduction is desired. In the case of direct reproduction the electroscopic marking particles are deposited in the latent image areas, whereas in the case of reversal re-production the particles are deposited outside the latent image areas.
Such prior art methods of development fall into two distinct categories, namely the so-called dry development method and the so-called liquid development method. Both these methods are widely known.
Dry development involves the attraction of electroscopic marking particles or so-called dry toners to the surface bearing the electrostatic latent image, such electroscopic marking particles being applied in the form of a powder cloud or carried on a triboelectrically different carrier particle.
In liquid development the surface containing the electrostatic latent image is contacted with a so-called liquid toner which comprises a dispersion of electroscopic marking particles in an insulating carrier liquid having a volume resistivity is excess of 10.sup.9 ohm-cm and a dielectric constant less than 3.0. The electroscopic marking particles or toner particles are attracted from said carrier liquid to the electrostatic latent image and deposited on the surface containing said image. The electroscopic marking particles usually are comprised of pigment as coloring matter and resins or varnishes or oils which serve as dispersing aids, fixing agents and can also confer the desired polarity and charge or sensitivity onto said particles.
In both dry and liquid methods of development the image formed on the surface of the recording member can be fixed thereon or transferred onto another surface if so desired.
A more recent type of electrostatic toner has now been developed which departs from each of the previously mentioned toning methods. This more recent toner consists of a solid and liquid phase in combination which is so formulated that the solid phase at rest is in a heavily bonded form in which the developer material or toner particles are joined by bonds forming a matrix or bonded flocculent structure, whereby the flow properties of the composition become non-Newtonian and whereby the developing material when applied to a surface containing a latent electrostatic image to be developed is restrained from migration to the surface to be developed and deposition thereon due to attraction by forces associated with the electrostatic latent image contained on said surface until sufficient shear exists to free the toner particles for development.
Such toners are essentially dispersions of a solid particulate phase in a liquid phase. The particulate solid phase and the liquid phase are so selected that the particulate matter is dispersed in a strongly flocculated state, hereinafter called the first state. In this first state the flocculated particulate matter forms a matrix, that is to say the particles are substantially linked on bonded to each other and the liquid phase is contained substantially within such matrix and surrounding same. In this first state the toner dispersion possesses non-Newtonian pseudo-plastic or plastic or thixotropic flow properties, that is to say the dispersion requires a certain applied shear force before Newtonian flow occurs, in which state the solid particulate phase is deflocculated, this state being called henceforth the second state.
When the above described toner is applied in said first state to a surface containing an electrostatic latent image by for instance pouring over the surface or by means of an applicator roller without any pressure, that is to say when there is no shear stress applied to the toner except some stress due to gravity, we have found that such toner cannot be used or considered as a toner in the sense of the prior art definition in that the electrostatic latent image contained on said surface is not developed at all, or if so, only to relatively low density, whilst the whole surface including the background area becomes heavily coated by the toner adhering to the surface and drying rapidly thereon.
If, however when applying the toner in said first state to a surface containing an electrostatic latent image, a shear stress of appropriate magnitude is applied simultaneously with or subsequently to the toner, for instance by means of roller pressure, and the toner is moved over said surface whilst under the influence of said shear stress, we have found that the electrostatic latent images contained on said surface are developed to high density, whilst the background areas remain free of toner deposit and furthermore the liquid phase wets the surface only superficially and consequently in cases where, for instance, the surface is contained on an absorbent substrate such as paper, liquid penetration into same and liquid carry-out are only slight.
Such toners provide a method of developing electrostatic latent images contained on a surface, such as for instance the surface of an electrophotographic or electrostatic recording member, in which method a toner dispersion is used which comprises a liquid phase and a dispersed solid phase consisting of electroscopic marking particles in a first state in which the flow properties of the toner are non-Newtonian and the electroscopic marking particles comprising the solid phase are flocculated forming a matrix which is structurally strong enough to prevent the extraction from it of individual electroscopic marking particles, which in this state are parts of the matrix, by attraction to the electroscopic latent image. If now a shear stress is applied to the toner, for instance by means of roller pressure, the toner is converted from the first state to a second state in which the flow properties of the toner become Newtonian and the solid phase becomes deflocculated. When this occurs the matrix is broken up into individual electroscopic marking particles which now form the dispersed phase, and in this state the electroscopic marking particles can be attracted by the electroscopic latent image and deposited onto the surface to give highly effective development of the image.
It has been found that this last mentioned toner type possesses many advantages over the earlier prior art types previously described in relation to electrostatic latent image development characteristics. Thus using this type of toner composition, hereinafter referred to as plastic flow toners, image development is very fast, solid fill-in is readily achievable, and continuous tone development is readily achievable. Because of the structure of plastic flow toners, background staining of developed surfaces is negligible, and solvent carry-out by the developed surface is minimal. However the low solvent carry-out which is a feature of plastic flow toners impairs their use in processes in which the developed image deposit is required to be transferred to another surface after development.
The present invention teaches a method whereby plastic flow toners may be used to develop images on plain paper surfaces and the like under the control of an electrostatic latent image.